Improvement in bridges



UNITED STATES JAMES B. EADS, OF ST. LOUIS, MISSOURI,

IMPROVEMENT IN BRIDGES.

Specification forming part of Letters Patent No. 144,519, dated November11, 1873; application filed August 27, 1873.

sists in a method of relieving the central piers of such bridges fromthe unequal thrust of the arches caused by the unequal loading of thesame. I effect this by making the roadway substantially continuousagainst compressive strain from the abutment on one end of the bridge tothe abutment on the other end of the bridge, so as to bring thehorizontal strain of all the arches ultimately on the said abutments.The roadway may also be made continuous in such manner as to resisttensional strains from pier to pier or abutment, and thus lessen thecompressive strains that would otherwise come against the abutments, thehorizontal resistance or chords being provided as gear the extreme sidesof the roadway as possi- In the drawings, Figure l is a side view of abridge with three arches, illustrating my improvement. Eig. 2 is a sideview enlarged of one end of an arch, part being in longitudinal sectionto show the compressive screw between the skew-back and chord ordistance piece. Fig. 3 is a horizontal section of one end of the arch,looking downward.

Suppose two spans constitute a bridge, and the shore-abutments alone areconstructed of sufficient strength to resist the thrust of the archeswith their loads. If the loads on both arches be. equal, and the archesof the same form and weight, the thrust of one span against the centralpier will be balanced by the thrust of the other, and a pier or columnsimply calculated to sustain the vertical pressure, as in ordinarytrusses, would be sufficient to insure stability; but if the load beremoved from one span, the pier may be overturned by the thrust of theloaded one. To prevent this overthrow of the piers thus designed, bysuch horizontal force, without the expense of increasing the size andweight of the piers, I

propose forming the chords of the wind-trussing of the roadway, and suchother longitudisary, of some non-expansible material, (wood,

for instance,) and so abut these chords and longitudinal members againstthe abutments and piers, in or about the line of the springing of thearches, as to cause the chords and members aforesaid, in the unloadedarches, to resist, by their compressive strength, the horizontal forceresulting from the loading of any arch in the system. These distancepieces or chords in any one arch will not be in compression at the timethe load is borne by that particular arch, except when initialcompression is produced in them, as hereinafter stated; but if they werefirmly secured to the piers, or pier and abutments supporting the arch,so as to resist tension, and were constructed throughout their length soas to resist tension, it will be seen at once that the compressivestrain thrown on the chords in the other unloaded arches would belessened by the amount of tension borne by the chords in the loadedarch.

The objection to combining timber with metal in the supporting membersof a truss does not apply in this method. In the truss one chord(usually the top or compressive member) is sometimes made of timber; butwhen this becomes decayed or injured it is scarcely possible to replaceit,save at the expense of constructing scaffolding, &c., to sustain theentire truss during repair.

In the proposed method any chord orpart of a chord may be readilyremoved without disturbing the arches, piers, or other portion of thestructure; for it will be seen that if an equivalent weight were put onthe floor-beams of any one arch its chords could be removed, and thestabilityof the arch would not be at all afl'ected so long as the chordsremained intact in the other spans of the series. Itis only when thespans are unequally loaded that any strain at all is upon the proposeddistance pieces or chords, (except as hereinafter stated,) whereas, inall forms of trusses whatever, the chords are constantly under strain,and hence repairs in them are ahnost impossible, except at greatinconvenience and expense.

In the drawings, A A A represent three chords or distance pieces, whichwill likewise serve as chords for the wind-trussing. B B.

are transverse floor-beams, and with the diagonals G G and the chords AA constitute the system of windbracing. D is a strong flat platesupporting the skew-backs E E and connecting the columns F, which standin transverse couplesor series, and constitute the piers supporting theends of the two arches. G Gr are iron screw-straps joining the woodenchords to the brackets H H on the skew-backs. I I represent lugs orbrackets on the plate D, to which may be secured stays, to secure thelower part of the arch against lateral movement, where the wind-bracingis omitted, to give head room for the roadway. K K arecompression-screws, which pass'through the brackets H H, and areprovided with nuts L L, which bear against the sides of the bracket, andby which the screw can be caused to press with greater or less forceagainst the metallic cap M at the ends of the distance piece, for apurpose hereinafter fully explained.

When an arch is loaded the distance pieces in the other arches will becompressed, and consequently slightly shortened. This will have theeffect of slightly changing the position of the tops of the piers, andif this change be so great as to endanger the stability of the piers,the longitudinal movement of the bases of the arches. may beaccommodated by arranging the skew-backs and plates D (or other platesupon those D) to slide on the tops of the piers or plates D in themanner of ordinary trusses, in which case the distance pieces would abutagainst the skew-backs.

Where several arches constitute a series of spans, and'theload be on onespan, the extent of shortening in all of the chords of the unloadedspans from the horizontal force of the load may cause the bases of theloaded arch 'to move so far apart as to cause objectionable deflectionin that arch. To prevent this, stronger abutments are provided, and theadjusting-screws K K, placed at the skew-backs of each arch, are made toact against the ends of the distance piecesin such manner as to producea compressive strain throughout the entire line of chords from abutmentto abutment when the bridge is unloaded. This initial compressive strainmay be produced to such an extent (as determined by calculation) as tobe just sufficient to relieve the chords of an arch entirely ofcompressive strain, when bearing the maximum load, and yet not producetension in it. In such case the deflection of the arch will be limitedby the compression in the arch itself, (due to the load,) and by theseparation of the arch bases to the extent of the initial compression inits own chords or distance pieces. In a series, for instance, of fivearches of, say, five hundred feet span, where the maximum horizontalforce produced by the load on one arch equals five hundred tons, if aninitial compressive strain of four hundred tons be produced in theentire system of chords from abutment to abutment, by the the screws,when the bridge is unloaded, then this initial strainwill be taken outof the chords of an arch next the abutment so soon as it bears itsmaximum load, while the compres sive strain in the remaining chords willbe proportionately increased. When all the arches are loaded the strainon the abutments will be five hundred tons, plus fourhundred tons, plusthe horizontal force of the unloaded arches say, three hundredtons-making a total of twelve hundred tons. This system is applicabletoall of the ordinary forms of truss-bridges, also; and by it the usualiron chords or tension members of a series of trusses may be dispensedwith, and wooden compression members or chords substituted, thuseffecting an important saving in cost.

It must be borne in mind that as each arch, in a uniform system,balances the thrust of its neighbor, the horizontal force against theabutments remains the same, whether there be one arch or any greaternumber of arches in .the same series. The same remark applies to trussesalso.

Owing to the expense and difficulty of joining a long series oftimbersin such manner as to bear the tensile strain throughout thelength of the series, I prefer the method proposed of increasing thestrength of the abutments, and producing by screws, or theirequivalents, as described, an initial compressive strain, and thusavoiding tensile strains in these chords. It may, however, be deemeddesirable to have provision in them for a small degree of tension, as incase of tornadoes, where they act as chords to the system of wind orhorizontal bracing between them; and to this end the timbers are joinedto the skew-backs by bolts and nuts, as shown in Figs. 2 and 3, toresist tensile strains. Where these fastenings are necessary the timbersforming the chords must be secured also throughout the length of thechords to each other by splicing or breaking joints, or other usualmeans for similar purposes.

I claim herein as new and of my invention- 1. The described method ofrelieving the central piers of bridges from the unequal thrust of thearches, or equivalent members of trussbridges, caused by moving loads,by making the roadway or longitudinal members supporting the samesubstantially continuous against compressive strains from the abutmentat one end of the bridge to the abutment at the other end of the bridge,substantially in the manner and for the purpose set forth.

7 2. In combination with the chords or distance pieces A A, thecompression and tensional screws K and G, substantially as and for thepurpose set forth. I

J AS. B. EADS. Witnesses:

SAML. KNIGHT, GEO. C. FABIAN.

